How to Make Extra Strong Plaster of Paris for Casting

By Updated Jul 19, 2020
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Plaster of Paris is great for sculptures.
Image Credit: kristo74/iStock/GettyImages

Plaster of Paris is created from fine ground gypsum that has been heated to 160 degrees, a process called calcining. The heating process removes moisture from the gypsum, altering its chemistry and creating a fine powder. When mixed with water it can be manipulated in many ways, from sculptures to modeling, but a basic plaster of Paris mixture is hard but fragile when dry. Strengthening it with glue creates a strong plaster that withstands the test of time.

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Basic Plaster and Glue Recipe

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This basic plaster and glue recipe is for small plaster casting projects. The mixture makes a hard plaster of Paris that's durable enough to be sanded with fine grit sandpaper. You can use the same recipe as a foundation for larger projects by scaling up the batch, as long as you make sure to keep the ingredient ratios the same.

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Things You'll Need

  • Newsprint or old newspapers

  • 8-ounce measuring cup

  • 1/4-cup measuring cup

  • Plastic mixing bowl

  • Water

  • White glue

  • Wooden spoon

  • Plaster

  • Face mask

  • Gloves

Mixing Strong Plaster

Step 1: Prepare the Work Area

Cover you work area with a layer of newsprint to prevent the plaster from sticking to important surfaces.

Step 2: Mix Water and Glue

Place 1 1/4-cup water in a plastic mixing bowl. Add 1/4-cup white glue to the water. Mix the water and glue until the glue is thoroughly incorporated into the water. The water should appear milky and have a creamy consistency.

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Step 3: Add the Plaster

Slowly pour 2 cups of plaster of Paris into the water. Add a small amount of plaster at a time. Sprinkle the plaster evenly across the surface of the water and glue mixture. Allow the plaster to settle to the bottom, and do not stir. Stirring at this point may cause excessive air bubbles that will weaken the plaster. Continue to add plaster of Paris until you've used all 2 cups.

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Step 4: Rest and Mix

Let the plaster rest for about five minutes before mixing. Use the plaster mixture immediately.

Tip

Using more plaster in your plaster mix creates harder denser molds. Using less plaster creates more porous and brittle molds.

Wear a face mask if the fine dust from Plaster of Paris affects you. Wear vinyl gloves to protect your hands.

Adjusting Drying Times

  • Speed up the drying time by adding a small amount of pure gypsum to the mix.
  • Add a couple of drops of lemon juice to slow the drying time.
  • Use warm water to speed up the setting time.

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Clean Up Hard Plaster

Cleaning as you go makes the process that much easier. Wash mixing utensils and bowls with warm water and soap. Never pour leftover plaster of Paris down the drain. This can clog pipes and become a costly mistake once it hardens. If plaster hardens on a surface, carefully break or chip the hardened plaster and clean the area with soap and water.

Warning

Mixing plaster of Paris with water causes an exothermic (heat-producing) reaction. Never place plaster of Paris onto skin. It can cause serious burns. If it gets onto your skin wipe the wet plaster away with a dry paper towel and clean the area with a wet paper towel and soap.

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